1. Field of the Invention
This invention relates to wear-resistant materials and more particularly to wear-resistant composite materials derived from refractory metals such as rhenium
2. Description of the Related Art
Material sciences seeks to exploit from available resources, namely those elements of the periodic table of elements, different materials having different characteristics that can be used for a variety of purposes and applications. Consequently, there is a great interest in developing conductors, insulators, soft materials, and hard materials from available substances and materials.
With respect to engines and mechanical components, wear resistance is typically a desired characteristic because it allows materials to last a longer time and consequently enjoy a cheaper cost per unit time. Even though a part may cost twice as much, it may last four times as long so is then approximately half as expensive as a cheaper part.
One particularly useful metal is elemental rhenium. Rhenium has atomic number 75 in the periodic table of elements. It is known as a refractory metal which means it has a very high melting point as compared to other materials. Despite the fact that it has a very high melting point, rhenium is not necessarily wear-resistant. Rhenium metal melts at approximately 5756xc2x0 F. (3180xc2x0 C., 3453xc2x0 Kelvin). However, rhenium begins to oxidize at a much lower temperature, at approximately 1000xc2x0 F. (538xc2x0 C., 811xc2x0 K). Rhenium retains excellent ultrahigh temperature strength of approximately 6-9 Ksi (kips per square inch, 1000 pounds per square inch) at 4000xc2x0 F. (2204xc2x0 C., 2477xc2x0 K). Far before it reaches this temperature, the oxidation temperature of rhenium becomes a factor and the oxidation of rhenium produces a volatile oxide that will continually evaporate from the surface of the rhenium part until the part has entirely vaporized.
This can lead to catastrophic failure as can be readily perceived as the evaporation of a part during operation would be much like the boiling away of a part made of dry ice. However, where oxygen is lacking, rhenium has been shown to retain high strength and to resist severe corrosion and wear both in established literature as well as independent tests. Such properties are present at room temperature operation and remain with rhenium even though the temperatures are elevated.
In prior literature, relevant alloying includes development of phase diagrams for several binary alloys of rhenium as well as rhenium-enhanced alloys such as those based on tungsten where rhenium is added to the matrix to enhance resulting properties. In such prior literature, rhenium has been a part of binary alloys such as in tungsten to increase ductility.
Rhenium has also been studied for its effect on chromium-based alloys. For steel, alloys with chromium are known that alloy the creation of xe2x80x9cstainless steel.xe2x80x9d When alloyed with chromium in amounts greater than 11%, steel converts from a highly oxidation prone substance to a more corrosion-resistant alloy with the general elimination of rust. Similar concepts are known for use in xe2x80x9csuper alloysxe2x80x9d and corrosion-resistive alloys.
In order to further enhance the wear resistance of a resulting alloy, a hard second phase can be introduced into the metal matrix of the alloy to produce a metal matrix composite. Such metal matrix composite (MMC) may use ceramic fibers such as those from alumina and/or silicon carbide to reinforce metal alloys based on such metals as aluminum and titanium. The m resulting materials are stiffer and lighter than the parent alloys and also have a high resistance to wear. Beyond the provision of a basic alloy, additional wear resistance can be introduced into metals by providing an additional second phase in order to increase the wear resistance.
However, the prior art does not well reflect the establishment of oxidation-resistant rhenium-based alloy. Consequently, as there is always a need for better materials and materials having better wear characteristics, the present invention provides a solution to a need that will always be felt for better materials.
Additionally, the art has not well addressed the wear that concentrates at small asperities which are microscopic metal protrusions that generally cause roughness on a surface such as a cast or focused metal. With respect to rhenium-based alloys, wear could be focused on such asperities and such wear would by friction create significant heat that would cause an alloyed rhenium to oxidize and vaporize.
In view of the foregoing disadvantages inherent in the known types of alloys, materials, and metal matrix composites (MMCs) now present in the prior art, the present invention provides a family of new wear-resistant rhenium-based MMCs wherein the same can be used in environments where unalloyed or pure rhenium would be subject to oxidation and/or vaporization while providing an increased degree of wear resistance.
The general purpose of the present invention, which will be described subsequently in greater detail, is to provide better materials for use in mechanically or otherwise stressful operating environments in order to provide better wear characteristics and function which are not anticipated, rendered obvious, suggested, or even implied by any of the prior art alloys and MMCs, either alone or in any combination thereof.
The present invention takes a refractory metal, particularly rhenium, that is subject to oxidation well below its melting point and well below the temperature at which it loses strength and strengthens the metal by selective combination to achieve an alloy that better resists oxidation and that has better wear characteristics. The refractory metal is combined with other alloying materials, such as metals, that have a strong affinity for oxygen. Such alloying materials include the metals chromium, cobalt, nickel, titanium, thorium, aluminum, hafnium, and related elements on the periodic table. These particular elements are at least somewhat soluble in rhenium, if not completely soluble.
It is believed that such elements protect rhenium by forming an oxide on its surface. Once the oxide has formed on the surface of the alloyed part, further attack by oxygen is prevented by the oxide coating. This is a phenomenon that is well-known with aluminum that oxidizes in oxygen, but once the surface of an aluminum object has oxidized, further oxidation is prevented as oxygen can not get to the unoxidized aluminum below the oxidized surface layer.
Consequently, those metals or other alloying agents that oxidize to form high boiling point oxides may well aid in the protection of the underlying rhenium alloy.
Using powder metallurgy, and possibly using free form fabrication (FFF) or casting, parts and components can be fabricated from rhenium-based alloys having oxygen-attracting qualities. The use of such techniques may render useful alloys with better wear and lower oxidation characteristics.
Additionally, the addition of a second phase in order to create a metal matrix composite (MMC) is disclosed in order to enhance the wear resistance of the resulting composite. The second phase may be silicon carbide but tungsten carbide, titanium carbide, boron carbide and the like will also enhance the matrix alloy""s wear resistance properties. These second phase materials may be in the form of continuous fibers, short fibers (or whiskers), particulates/particles. Preferably, the second phase in the form of granular particles as it is believed that this form of the second phase better enhances the wear resistance properties of the resulting composite and makes formation easier. Other features and advantages of the present invention will become apparent from the following description of the preferred embodiments which illustrate by way of example the principles of the invention.